Figure 1: Use of carbon fiber enables a stronger, lighter and thinner containment sleeve, thus enabling air gap reduction. Composites also reduce losses in the sleeve, improving machine performance. Source: Arnold Magnetic TechnologiesFigure 1: Use of carbon fiber enables a stronger, lighter and thinner containment sleeve, thus enabling air gap reduction. Composites also reduce losses in the sleeve, improving machine performance. Source: Arnold Magnetic Technologies

High efficiency, power dense DC motors typically use rare-Earth permanent magnets, the most efficient of which use magnets mounted to the rotor. In low-speed applications, magnets may simply be bonded onto the shaft, but high performance motors that spin up to extreme rotational speeds — increasingly configured in a Halbach arrangement — need to be retained externally to keep them in place.

The traditional retention method is to use a sleeve made out of materials such as Inconel or stainless steel, and this extra sleeve mass might be seen as a necessary tradeoff to working with such high-performance motors. However, with the push for electrically powered aerospace applications, electric motorsports advancements, high-performance consumer vehicles and more, the potential use of carbon fiber for low-mass magnet retention provides an exciting opportunity.

Carbon fiber: Lightweight, low inertia option

As a material, carbon fiber provides an extremely high strength-to-weight ratio, and has been proven out in a variety of well-known applications, such as aerospace components, motorsports and even bicycle frames. Arnold Magnetic Technologies has been developing composite contained rotor assemblies for electrical motors for seven years, and continues to push the envelope of what is possible with this technology.

Replacing the metallic sleeve with a carbon fiber composite for retention means a lighter weight motor assembly, lower rotational inertia and greater compressive pressure. The use of carbon fiber also allows for a thinner air gap between the shaft assembly and stator, and reduces self-load due to centrifugal forces. Electrical losses in the sleeve are still present, as carbon fiber is axially conductive, but are significantly lower than in the metallic sleeve it replaces.

Methods for carbon fiber encapsulation include:

  • Press fit sections of pre-made carbon fiber tubing on the magnetic rotor assembly (necessarily means complexity in tolerancing, controlling stresses when press-fitting and the potential necessity to use a series of press fit rings)
  • Hydraulic inflation of a pre-made sleeve (not ideal due to additional strain on the resin)
  • Direct wind carbon fiber onto rotor assembly (Arnold Magnetic Technologies’ preferred solution, for a number of reasons)
    • Ground diameter tolerance is not critical (from a fit perspective) as the fiber is wound directly onto the surface
    • The sleeve is developed between collars that are integral to the rotor, meaning end faces are both supported and contained
    • The are no cut fibers, minimizing any structural risks within the sleeve

While motor manufacturers may not have the in-house expertise to direct wind carbon fiber themselves, Arnold Magnetic Technologies can work with customers to assemble and balance their rotors as a pre-made assembly. This takes the burden of implementing this process out of the hands of motor manufacturers, producing a ready-made solution for motor assembly.

Carbon fiber motor shaft assemblies are cutting-edge today. As such, Arnold Magnetic Technologies can develop bespoke high-performance solutions tailored to customer needs. They can then work with customers to turn these one-of-a-kind super-motors into the production super-motors that will enable high-performance vehicles, robots, drones, aerospace applications and more well into the future.

Arnold Magnetic Technologies

Arnold Magnetic Technologies is a leading global manufacturer of high-performance permanent magnets, flexible composites, electromagnets, magnetic assemblies and precision thin metal foils. With worldwide manufacturing, a 125 year-plus history, and a new technology center in Madison, Wisconsin, they stand ready to further the development of carbon fiber in motor manufacturing.

Find out more about their carbon fiber magnet encapsulation solutions here, or see this tech talk for a more in-depth briefing on the subject.